Abstract: With glutaraldehyde as finishing agent, the surface of an up-conversion luminescent material, NaY_0.57Yb_0.39Er_0.04 F₄, was modified with aldehyde groups by surface grafting modification. FI-IR proved that aldehyde groups existed on the surface. The quantity of aldehyde groups was 1.38 ×10³ mol·g^-1 which was detected by UV. Qualitative TGA indicated that there were organic groups on the surface. The particle diameter of the modified up-conversion luminescent material was aggrandized in SEM. In settlement testing, the modified up-conversion luminescent material has more favorable dispersion stability than un-modified materials.